We have used recombinant DNA techniques to construct a series of eucaryotic vectors. These vectors function both in E. coli and in mammalian cells and, thus, may be used to isolate defined eucaryotic genetic segments in E. coli for subsequent transfer to and functional analysis in an appropriate mammalian genetic background. The prokaryotic elements of the constructs derive from E. coli chromosomal DNA, bacteriophage lambda and the plasmid pBR322; the eucaryotic elements derive from simian virus 40. We are currently using this approach to study mammalian promoter regions and RNA processing segments in in vitro tissue culture systems.